206 E. W. Yemm 



the time scale of these changes varies widely ; the rise in C0 2 

 production and rapid yellowing occurred after about 25 days 

 in Cherry Laurel and after 8-10 days in Tropaeolum leaves. 

 With mature leaves of barley and of grasses the changes are 

 much more rapid and yellowing often occurs 2-3 days after 

 detachment from the plant. Despite these wide differences in 

 the rate of ageing, many of the same features can usually be 

 recognised; during yellowing relatively high or rising rates of 

 respiration are generally observed with detached leaves of 

 different species. More limited data are available concerning 

 the respiration of leaves when they undergo normal senes- 

 cence while still attached to the plant. However, Arney 

 (1947) in experiments with attached strawberry leaves has 

 shown that an appreciable rise in rate of C0 2 output occurs 

 during yellowing. 



A detailed interpretation of the respiratory drifts in ageing 

 leaves will not be attempted here. There is strong evidence 

 that the respiratory mechanisms are considerably modified; 

 with detached leaves of barley and other species, chemical 

 analyses indicates that during yellowing a substantial part of 

 the C0 2 lost from the tissues is formed by the oxidation of the 

 carbon skeletons of amino acids. Godwin and Bishop (1927) 

 found that a loss of relatively stable glycosides and probably 

 polysaccharides took place during the senescence of Cherry 

 Laurel leaves. High rates of respiration seem, therefore, to 

 be closely related to the breakdown of proteins and other 

 complex constituents in senescent leaves. Soluble metabolites, 

 such as amino acids formed in this way, may be important 

 respiratory substrates which sustain the cellular oxidations 

 associated with the rapid release of C0 2 from ageing tissues. 



In addition to leaves, some other senescent plant organs 

 show an acceleration of respiration during ageing. The so- 

 called climacteric rise, observed in apples when they ripen 

 and become yellow (Kidd and West, 1930), has been shown to 

 occur in many other ripening fruits. The metabolic changes 

 which take place in apples have been extensively studied (see, 

 for example, the review by Pearson and Robertson 1954); 



